Lime is often used as the preferred reagent for neutralization of acidic compounds or in other chemical applications. However, lime reacts with sulfates, carbonates and bicarbonates to form insoluble compounds which result in scaling of equipment and piping. It has a low solubility making solution storage impractical and requiring either weigh feeding of powder or slurry storage and metering of slurry. It is a finely divided powder which is difficult to handle and creates excessive dusting.
It is known that when insoluble compounds are precipitated from solution they tend to crystallize on surrounding surfaces. Most compounds supersaturate and a finite time is required for precipitation to reduce the supersaturation. The phenomenon of precipitation on lime slurry particles has been referred to as lime stabilization. To date the potential of these effects has not been fully utilized. The invention provides an integrated process which is performed in a automated sequence in specialized equipment to mitigate the dusting problems, eliminate scaling and also provide for trouble free metering. It is best described with a comparison to the conventional lime addition systems employed in treating of municipal water supplies.
The U.S. Environmental Protection Agency (EPA) has established maximum lead and copper contaminant levels in the National Primary Drinking Water Regulations. These regulations are expected to reduce the exposure of approximately 130 million people to lead in drinking water. In 1% of the municipal water systems there is excessive lead or copper in the source water. In the remainder of the municipal water systems the source of the excessive lead or copper contamination is the result of chemical solution of lead and copper components of the piping systems. In order to meet the Maximum Contaminant Level Goals most of the municipal water systems will be required to install some sort of corrosion control treatment.
The copper and lead corrosion experienced in water systems is primarily caused by the acidity of the source water. This acidity results from mineral acids in acid rain and dissolved carbon dioxide which forms carbonic acid. Corrosion control can be accomplished by pH and alkalinity adjustment to reduce the acidity of the water, calcium adjustment to promote the formation of protective coatings inside pipes and plumbing or addition of a phosphate or silica-based corrosion inhibitor to form a protective coating inside of pipes and plumbing.
Of the corrosion treatment options the most widely practiced is reduction of the acidity with an alkali. The three alkalies commonly used are calcium hydroxide, potassium hydroxide and sodium hydroxide. Selection of the alkali to be used involves consideration of the effectiveness of the reagent in reducing corrosion of piping components, ease of control of pH, the capital and operating costs of a treatment facility, the effect on the taste of the water, the possible health effects resulting from the introduction of the reagent into the water and the safety in handling the reagent.
From a technical standpoint calcium hydroxide is the preferred reagent. In addition to reducing the acidity a protective coating is formed inside the piping system. Calcium neutralized waters have a greater buffering capacity, thereby providing a more stable pH and greater ease of control. Lime treated waters tend to taste better. Potassium hydroxide treatment tends to impart a bitter taste to the water. Sodium hydroxide treatment has the disadvantage of raising the sodium content of the water making it a health concern for certain individuals and is often dropped from consideration for this reason. Of the three reagents calcium hydroxide is the least hazardous to handle. Addition of lime does increase the hardness of the water, but since acid waters generally are low in hardness the effect of this increase is negligible.
A financial comparison of acid water treatment costs given in Table 1 shows substantially less reagent cost for lime or calcium hydroxide. The difficulties in handling calcium hydroxide in the conventional systems increases the capital costs and the operating and maintenance costs to the extent that overall calcium hydroxide treatment is the most costly. While lime treatment is the preferred process many water suppliers are going to the use of other reagents because of the operating difficulties and high maintenance costs associated with lime use in conventional systems.
Conventional lime treating facilities usually consist of a covered storage area used for storing, for example, fifty pound bags of hydrated lime, a feed hopper into which the hydrated lime is manually transferred, a weigh feeder or volumetric feeder which meters the hydrated lime into a dissolving tank and a pump for injecting the lime solution into the water main. The systems are designed to operate unattended. In large installations quicklime may be used instead of hydrated lime but this requires the addition of slaking equipment and continuous on site supervision.
Handling the finely divided lime powder creates a severe dust problem and generally necessitates a separate building for the process equipment. Filling the feed hopper and metering the powder into the dissolving system are dusty and labor intensive operations. The feed hopper and feeder are subject to frequent plugging. Scale formation in the dissolving tank and associated piping is a continuing occurrence causing line plugging and injection pump malfunction. These problems result in excessive operating and maintenance costs. In municipal water systems having multiple, widely separated well sites requiring on site treatment the addition of lime requires substantially more capital for buildings and the maintenance and operating problems are increased. The operating and maintenance problems affect the reliability of these systems to the extent that State regulatory agencies are reluctant to issue permits for new calcium hydroxide treatment systems.
Use of the present invention eliminates all of the aforementioned disadvantages of lime handling and results in being the most economic means of water neutralization.
TABLE ______________________________________ FINANCIAL COMPARISON OF pH TREATMENT SYS- TEMS FOR A 272,000,000 GALLON PER YEAR MUNICI-
WATER SYSTEM SUPPLIED BY FIVE WIDELY SEPARATED WATER WELLS. CONVENTIONAL TREATMENT SYSTEMS IMPROVED Calcium Sodium SYSTEM Hydrox- Potassium Hydrox- Calcium ide Hydroxide ide Hydroxide ______________________________________ Reagent Cost 3,000 19,800 11,600 3,000 Operating and 25,500 5,000 5,000 5,000 Maintenance Labor Debt Service 125,100 112,200 112,200 48,800 on Capital Facility (8% for 15 Years) Total Annual $153,600 $137,000 $128,800 $56,800 Operating Costs ______________________________________